In ion implantation systems, an ion beam is directed towards a workpiece (e.g., a semiconductor wafer, or a display panel) to implant ions into a lattice thereof. Once embedded into the lattice of the workpiece, the implanted ions change the physical and/or chemical properties of the implanted workpiece regions, relative to un-implanted regions. Because of this ability to change material properties, ion implantation can be used in semiconductor device fabrication, metal finishing, and various applications in materials science research.
During a typical implantation process, the ion beam has a cross-sectional area that is significantly smaller than the surface area of a workpiece to be implanted. Because of this, ion beams are scanned over the surface of the workpiece to achieve a specified uniformity of doping profile in the workpiece, where the doping profile consists of a desired depth distribution at a desired volumetric concentration. For example, FIG. 1 shows an end view of a conventional ion implantation system 100 where an ion beam 102 traces over a scan path 103 to implant ions into the lattice of a workpiece 104. During this tracing, the ion beam 102 is often scanned over a first axis 105 while the workpiece 104 is mechanically translated over a second axis 106. However, in other embodiments the beam could also be scanned over both the first and second axes 105, 106; could be magnetically and electrically scanned over the axes 105, 106, respectively; and so on.
In practice, as the ion beam 102 traces over the scan path 103, the shape and/or cross-sectional area of the beam can vary, such as shown in FIGS. 1B-1F, for example. FIGS. 1B-1F show the ion beam 102 scanning across the workpiece 104, where the width of the beam can be larger (e.g., more diffuse) near the center of the workpiece (central width Wc in FIG. 1D) and smaller (e.g., more focused) near the edges (e.g., left and right widths, WL1, WR1 as shown in FIGS. 1B, 1F, respectively). If these variations in beam width and/or associated current density are not accurately measured and accounted for, the uniformity of doping profile actually formed in the workpiece 104 can differ from the specified uniformity. Such non-uniformity can result in the implanted workpiece yielding fewer functioning electronic devices than desired.
One underlying cause of such beam variations can be the so-called zero-field effect (ZFE), which may also be referred to as zero field anomaly (ZFA). ZFE often occurs when the magnitude of a scanning field, either electric or magnetic, approaches zero, thereby causing a sudden “spike” or “dip” in beam current, while the zero magnitude scanning field is applied. FIG. 2A shows an example of a beam scanning waveform 204, which can be used to scan the ion beam back and forth over the scan path (e.g., as shown in FIG. 1). As can be seen when viewing FIG. 2A-2B simultaneously, when the beam scanning waveform 204 is near zero (206 in FIG. 2A), a sudden spike in beam current 202 (FIG. 2B) can occur. Absent countermeasures, this beam current “spike” 202 can cause the part of the workpiece encountering the ZFE to be implanted differently from specified, resulting in detrimental non-uniformity on a workpiece.
The exact cause of the ZFE is not clear, however it likely has to do with beam neutralization, i.e., the transport enhancement that occurs when the space-charge of the ion beam is cancelled by a medium with opposing electric charge in a beam line, such as for example a neutralizing beam plasma generated via collisions of beam ions with the neutral background gas. The ZFE may be the result of the magnetic field, or the induced electric field (e.g., due to the time varying magnetic field), forcing neutralizing electrons out of the beam line area (e.g., the magnetic or induced electric field acts upon the electrons with a force that pushed them out of the beam line) and thereby reducing charge neutrality and leading to transport enhancement or reduction (e.g., providing more or less beam current depending on how charge neutralization affects beam transport). However, regardless of the cause of ZFE, the result of the zero field effect is an uneven beam current profile that may result in a non-uniform implant on the workpiece.
Accordingly, aspects of the present disclosure are directed toward improved ion implantation systems that mitigate ZFE.